Forming helical strip material is well known and many arrangements for carrying out this method have been desribed. However, the previous methods have not been applicable to twisting a thin material which, although rigid, is for example of a fairly soft material such as aluminum.
An example of a twisting apparatus and method is shown in U.S. Pat. No. 1,740,612. This discloses an apparatus for twisting an I-shaped cross section copper strip which is to form the core of a cable. Because the core is to protect the cable it has to be of a substantial cross section and therefore relatively stiff and can be handled with much less difficulty than thin strips of aluminum. The untwisted strip mounted on a roll 8 is passed between two rolls 56, the roll 8 being rotatable about the axis of the machine. Although the rolls 56 are driven, the main force which drives the strip between the rolls 56 is provided by means of a constantly driven feed wheel or standardizer 10 which draws the twisted material through the rolls 56. Such an arrangement will only be applicable to a relatively stiff strip of material. Also as the roll 8 has to be rotated about the axis of the machine the size of the roll 8 is limited and hence the amount of strip material which can be supplied in one run of the apparatus is severely limited.
Furthermore, there is insufficient control on the feeding of the strip to the rolls 56 for the apparatus to be applicable to very thin strip material.
Thus the apparatus of U.S. Pat. No. 1,740,612 would not be applicable to thin soft material as a strip of aluminum, for example, of a thickness of 0.5 mm.
Thus twisting of a strip of thin aluminum sheet material has been a difficult operation and is particularly difficult to carry out continuously.